1. Field of the Invention
The present invention relates to systems and methods for ocean thermal energy conversion and particularly to systems and methods that combine thermal energy conversion with a mariculture operation.
2. Description of the Prior Art
In recent years, rapidly depleting supplies of fossil fuels have led to searches for alternative ways to satisfy worldwide needs for usable energy sources. One area of investigation has been the conversion to useful work of the potential energy represented by the difference in temperature between warm surface water and deep water in tropical and sub-tropical ocean regions.
One of the earliest researchers in the development of practical means for ocean thermal energy conversion (conventionally abbreviated "OTEC" ) was a Frenchman, Georges Claude. U.S. Pat. No. 2,006,985, issued to Claude and Boucheret on July 2, 1935, describes a system for obtaining power from the difference in temperature between the surface water of tropical seas and the water at great depths. In the Claude system, sun-warmed surface seawater is vaporized at reduced pressure, using the sensible heat of the water to supply the latent heat of vaporization. The steam generated then drives a turbine and is subsequently condensed at a lower pressure by heat transfer to cold deep ocean water.
More recently, in U.S. Pat. No. 3,347,753 issued Oct. 17, 1967 to M. Morse, a seawater desalination plant was proposed in which power extracted from ocean thermal differences is used for pumping as well as desalinization. The Morse system uses a closed-loop secondary working fluid circuit to avoid the problem of having to remove continuously the large amounts of air dissolved in surface seawater in order to maintain the vacuum required for boiling to occur in a Claude-type system. The separate working fluid preferred by Morse is a liquid such as distilled water which will boil readily at the temperature of the surface seawater under reduced pressure.
A further OTEC refinement is presented in U.S. Pat. No. 3,928,145. This patent, issued to Donald F. Othmer on Dec. 23, 1975, discloses a closed cycle thermodynamic system which uses a separate working fluid, such as propane, ammonia, or n-hexane, to power a turbine generator. In the Othmer system the working fluid is evaporated in a flash boiler following heat interchange with warm surface seawater from an adjacent ocean region. After the flash evaporation step, which occurs without the addition of further heat, the working fluid vapor is expanded through the turbine to a lower pressure, at which it is condensed by transfer of its latent heat of vaporization to cold seawater pumped from a deep portion of the same ocean region.
The advantages of using ammonia or one of the other separate-loop thermodynamic fluids suggested by Othmer, instead of distilled water, are not only that no deaeration of large amounts of seawater is required but also that a smaller turbine may be used because of the higher pressure and density of the vapors.
The Othmer Pat. No. 3,928,145 provides an additional advantage, in that the cold, deep seawater used to condense the working fluid of the OTEC power plant is discharged to a mariculture operation, where its rich store of nutrients fertilizes marine plants in a preselected food chain which may include edible shellfish and crustacea.
The potential of the nutrient-rich deep waters of the ocean to increase fish production has long been recognized. In ocean regions where natural upwelling of these deep waters occurs, such as the coastal waters off Peru, the concentration of fish is so great that these areas, comprising only 0.1% of the ocean's surface, supply almost half of the total world fish catch.
Several investigators have suggested the use of artificial upwelling (i.e., piping of deep ocean waters to a selected mariculture site) as a means to increase manyfold the world's production of marine organisms for human and animal consumption. For example, in an article entitled, "Marine Farming" (Scientific American, Vol. 223, No. 6, Dec. 1970), G. B. Pinchot suggests pumping deep water into a central lagoon of an atoll. The lagoon would provide a catchment basin, retaining the nutrients of the pumped seawater at or near the surface.
Similarly, John D. Isaacs and Walter R. Schmitt of the Scripps Institution of Oceanography have written that some atoll lagoons, such as Kwajelein in the Marshall Islands, constitute nearly ideal areas in which nutrient-rich deep seawater could be discharged and confined for use in marine farming. (J. Cons. Int. Explor. Mer, Vol. 33, No. 1, Copenhagen, Nov. 1969).
In "Open Sea Mariculture", Joe A. Hanson, Ed. (Dowden, Hutchinson & Ross, Inc., Stroudsburg, Pennsylvania, 1975), several hundred coral atolls in the Pacific and Caribbean Oceans are considered to share basic characteristics suitable for mariculture. These characteristics include almost completely enclosed lagoons and proximity to deep water where phytoplankton nutrients are available. Among Pacific atoll chains cited as being of interest are: the Marshalls, Gilberts, Carolines, Marianas, Line Islands, and the Hawaiian Archipelago. With a suitable rate of nutrient input to an atoll lagoon, a high rate of plankton production should occur. These plankton permit the culturing of secondary marine organisms, such as oysters, mussels, clams, shrimp and the herbivorous finfish, for direct human consumption.
Marine species that have been recommended for cultivation in a marine farming operation include brine shrimp, Artemia salina, and various types of penaeid shrimp. In particular, the artificial culture of penaeid shrimp has been the object of lifelong study by Dr. M. Fujinaga (see, for example, U.S. Pat. No. 3,477,406 issued Nov. 11, 1969).
The advantage of combining a mariculture operation with an OTEC power plant is that revenue from several products can be optimized, thereby counteracting the inherently low thermal efficiency of an OTEC plant.
For example, the above-mentioned Othmer patent points out that the theoretical Carnot efficiency of a thermodynamic system operating between a typical warm seawater temperature of 86.degree. F (30.degree. C) and a cold seawater temperature of 41.degree. F (5.degree. C) is only 3.3 percent. As a means of increasing the thermal efficiency of an OTEC plant, the Othmer patent suggests additional solar heating of the warm seawater heat transfer medium in ponds with black bottoms and transparent covers, in various types of known closed tubular systems or pressurized solar heaters, or by dissolving suitable dyes in the water or covering the surface with charcoal, graphite or other black pigment to increase the absorption of solar radiation. By use of such techniques the upper temperature of the thermodynamic system can be raised enough to triple the theoretical Carnot efficiency.
An OTEC power plant large enough to supply the nutrient-rich deep water requirements of a typical atoll lagoon, however, would require such large quantities of warm seawater that solar heaters of conventional design would be impractically large and expensive. Furthermore, the use of a flash boiler, as in the Othmer system, results either in a substantial drop of temperature and pressure from the liquid to the vapor state, in the case of single-effect evaporation, or in high plant cost, in the case of multiple-effect evaporation.
Another economic drawback involved in combining conventional OTEC plants (which typically produce electrical power or desalinated water) with atoll-based mariculture operations is that atolls rarely have sufficient population to consume more than a small fraction of either the electricity or the potable water produced by the plant. Moreover, atolls are usually so far removed from large population centers that transmission of the electric power or shipment of the fresh water produced would not be economically feasible.
On the other hand, there exists a worldwise demand for fertilizer that far exceeds the available supply. An important fertilizer, which is also a basic ingredient for other fertilizers, is anhydrous ammonia. A major portion of ammonia production is by synthesis from hydrogen and atmospheric nitrogen. The usual source for hydrogen in ammonia synthesis plants is natural gas, the supply of which is rapidly dwindling.